Parallelism is a term that refers to how clauses, phrases, or words joined together in a list must have the same grammatical structure. For example, if you were to have a list of verbs, they would all have to be in the same tense, such as past, present, or future (e.g. "I like to run, jog, and swim."). Similarly, if you were describing someone, all the words in the list would need to be adjectives to maintain parallelism (e.g. "My cat is cute, lazy, and funny.").
For more specific examples and explanations of parallelism, please see the linked handout: